It is known in the plastic molding art to use pressurized fluid in conjunction with the plastic molding of articles, as disclosed in the Friederich U.S. Pat. No. 4,101,617.
Gas-assisted injection molding is a thermo-plastic molding process which provides stress-free large parts with a class A surface and virtually no sink marks. Gas-assisted injection molding is a low-pressure molding process compared to conventional injection molding. In this process, inert gas is injected into the plastic after it enters the mold. The gas does not mix with the plastic but remains in the middle of the thicker sections of the molding. By controlling the gas pressure, the quantity of plastic injected into the mold (short shot) and the rate of gas flow, a predetermined network of hollow interconnecting channels is formed within the molded part. The gas pressure remains constant in the network of hollow channels throughout the molding. This compensates for the tendency of the plastic to shrink at the thicker areas of the molding, preventing warpage and reducing stress. The gas pressure is relieved just prior to opening the mold. Because of the relatively low injection pressure, large parts can be molded with substantial reductions in clamp tonnage.
The gas system equipment provides the precise control of pressure, timing and volume of gas which is injected into the part, all of which are important to the control of the gas-assisted injection process.
In U.S. Pat. No. 4,948,547 entitled "Improved Method for the Use of Gas Assistance in the Molding of Thermoplastic Articles," assigned to the Assignee of the present invention, a method of gas-assisted injection molding is disclosed in which a charge of pressurized gas is injected into the mold but not into the article-defining cavity. The gas charge is of a predetermined quantity and pressure, sufficient to assist in filling out the article defining cavity with resin and promoting surface quality.
U.S. Pat. No. 4,935,191 discloses a process for producing an injection molded product including introducing a stream of plastic material into a mold space at a first pressure. A quantity of gas is stored in a storage chamber at a second pressure which is at least as high as the first pressure. The gas is introduced into the molten stream of plastic material immediately after the molten material has passed the position at which the gas is introduced, thereby forming a gas cavity in the molten material.
The mechanism for charging a pressurized fluid or gas for use in a molding process is described in detail in U.S. Pat. No. 4,855,094. Also, a detailed description of various mechanisms for introducing the pressurized gas into the mold is described in this patent which is assigned to the Assignee of the present application. One mechanism is a sliding needle valve located in a sprue bushing. In an open position of the needle valve, gas is injected through a port.
One limitation of the prior art is that a charge of pressurized gas is stored some distance from the mold and, consequently, there are pressure drops as the gas travels to the mold. Repressurization must then occur. Another limitation of the prior art is that unless some mechanical mechanism is provided, the pressurized gas must have a pressure less than the pressure of the molten plastic in the mold.